This invention relates to a thermally responsive switch suitable for protecting electric motors employed in hermetic compressors of refrigerating machines against burnout due to overheating, and more particularly to such a thermally responsive switch capable of reliably cutting off an electrical path in an abnormal temperature condition upon termination of the life of the switch.
An electric motor employed in a hermetic compressor of a refrigerating machine is usually driven in a hermetically sealed compressor housing with refrigerant and lubricating oil surrounding it. Taking into consideration the maximum pressure values in both low and high pressure conditions during respective compressor on and off periods, the pressure in the hermetically sealed compressor housing is varied in a vast range. A thermally responsive switch used in the above-described atmosphere is required to be reliably responsive to the changes in the motor winding temperature and the current and to open an electrical path in an abnormal condition so that the running of the motor is interrupted. In order to operate the thermally responsive switch as described above, its parts including movable and fixed contacts are enclosed in a hermetic casing so that invasion of the refrigerant or the like into the casing interior can be prevented. Furthermore, the hermetic casing of the thermally responsive switch is required to have a high level of pressure tightness. Furthermore, the thermally responsive switch is required to have a high level of thermal responsiveness and specific characteristic of distinguishing between a normal current and an abnormal current. Additionally, the thermally responsive switch is required to be small in size, large in the switching capacity and superior in durability while it should be stable in the quality and cost effective.
However, conventional thermally responsive switches are disadvantageous in that the thickness of the hermetic casing enclosing the switch elements needs to be largely increased so that a sufficient pressure tightness can be secured.
Furthermore, the position of the movable contact relative to the fixed contact needs to be checked by means of X-ray irradiation after the switch casing is hermetically sealed finally. Thus, checking the position of the movable contact relative to the fixed contact cannot be readily performed.
Furthermore, the service life of the thermally responsive switch terminates after a large number of times of circuit making and breaking operation for a long period. In this case the contact surfaces of the movable and fixed contacts are melted such that the contacts are welded together, resulting in continuous flow of the current into the motor. In such a case, insulators of the motor windings are deteriorated because of the heating of the windings and consequently, a large short-circuit current is caused to flow in the circuit, resulting in the increase in the atmospheric temperature in the hermetic compressor housing. As a result, a least durable portion of the casing, for example, a so-called cluster where terminals insulated by glass are hermetically held is caused to come out of the compressor housing and flames blow out, resulting in the possibility of an outbreak of fire.